Pellets for Se Encapsulation

ABSTRACT

Pellets encapsulating selenium or a compound of selenium comprise one hollow cavity filled with the selenium surrounded by a matrix which is able to form an eutectic with at least one of the constituents of a batch of molten raw materials for the manufacture of glass.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the entry into the United States of PCT Application Number PCT/EP2006/067502 filed Oct. 17, 2006 and claims priority from European Patent Application No. EP/05109699.8 filed Oct. 18, 2005, the entirety of each of which is hereby incorporated by reference.

SUMMARY OF THE INVENTION

This invention relates to pellets materials comprising selenium. More precisely, it relates to pellets that encapsulate selenium and/or at least a selenium compound. In particular, the invention relates also to pellets encapsulating selenium that can be incorporated to a batch mixture of components suitable for glass melting manufacturing processes.

According a first aspect, the invention provides pellets encapsulating selenium and/or at least one selenium compound as defined in claim 1.

According to a second aspect, the invention provides a process for the manufacture of pellets according to claim 7.

According to a third aspect, the pellets according to claim 1 are used as a selenium raw material for incorporating a glass molten batch according to claim 11.

According to a fourth aspect, the invention provides a method for reducing selenium burn off in a selenium comprising glass making process according to claim 12.

According to a fifth aspect, the invention provides a soda lime glass comprising selenium according to claim 13.

Dependent claims define further preferred embodiments of the invention.

The present invention may provide one or more of the following advantages:

-   -   Simple mechanical method of preparation of the pellets;     -   simplicity of incorporation of the Se, like any other         traditional glass component;     -   no requirement for any special and/or particularly expensive         dispenser or dosing equipment for the incorporation of the Se         component in a batch of raw materials;     -   access to new high Se glass formulae.

DETAILED DESCRIPTION

Pellets are small pieces of material of any shape, having at least 1 cm in their largest dimension. Most often, they have at least 0.5 cm in their largest dimension. Generally, they do not exceed 20 cm for that largest dimension. Preferably, pellets are of parallelepipedic, cubic or cylindrical shape. Pellets according to the invention are composed of two parts: one hollow cavity filled with a heart of selenium and/or at least one selenium compound and a matrix surrounding the filled hollow cavity, which hides and isolates the heart from the pellet environment. The matrix, according to the invention, is a compressed tablet that does not melt below 1300° C. under normal atmospheric pressure. The matrix must be free from any compound which could give rise to a gaseous compound when heated from atmospheric temperature up to 2000° C. It must as well be able to slowly form an eutectic product with at least one of the constituents of a batch of molten raw materials commonly used in a glass manufacturing process, when dipped into said molten glass raw materials batch. The eutectic product has a melting temperature of from 400 to 1800° C. under normal pressure.

The heart of selenium and/or at least one Se compound is selected from metallic selenium, inorganic and organic salts, as well as other organic compounds comprising selenium in their molecule. In particular, the selenium compound may be selected from the selenites and the selenates. For example, it may be a selenite of an alkali metal, like a sodium selenite (Na₂SeO₃).

The matrix may comprise any inorganic or organic compound, possibly mixed, able to encapsulate efficiently the heart of selenium and/or selenium compound. By efficiently we intend to mean able to mask for a time not lower than 5 to 15 minutes the presence of selenium and its compounds from hot molten glass batches components, like SiO₂, CaO, Na₂O, K₂O, Al₂O₃ and the like. According to the invention, the matrix may comprise at least one of the following compounds: SiO₂, Na₂O, K₂O, Li₂O, B₂O₃, CaO, MgO, BaO, ZnO, Al₂O₃, Fe₂O₃, Se, TiO₂, CoO, MnO, Cr₂O₃, CeO₂, ZrO₂, NiO, SnO₂, Cu₂O, V₂O₅ and SO₃. Those compounds may be present as such in the matrix, or mixed or combined with others in order to form special components like sand, cement of any type, dolomite . . . and the like. Two or more of them may as well be mixed together.

According to an embodiment of the invention, the matrix is generally able to form an eutectic with one or more of molten CaO, MgO, Na₂O, SiO₂ or Al₂O₃ present in a raw material glass batch.

As mentioned above, a second aspect of the invention is to supply a process for the manufacture of pellets according to claim 7. In that process:

-   -   part of the total quantity of the matrix material is dry mixed     -   the mixture is poured into a mould bearing an open cavity     -   a rod having a cross section slightly smaller than the one of         the cavity aperture is then inserted into the cavity and allowed         to lay on the matrix material mixture     -   the rod is then pressed with the aid of a press in order to         solidify the matrix material into a lower half pellet     -   the rod is removed and the selenium and/or selenium compound is         deposited at the top centre area of the half pellet     -   a second and last part of the quantity of the mixed matrix         material is poured over the selenium and/or selenium compound     -   the rod is put again in place inside the cavity of the mould and         pressed     -   the finished pellet is removed from the mold.

An embodiment of the process according to the invention consists of using two different rods, a first one with a plain section end for pressing the first lower half pellet and a second one with a small hole located in the centre of its section end in view to keep the selenium and/or selenium compound at the heart of the pellet during the pressing operation.

According to another interesting embodiment, a small proportion of a binding agent may be added to the selenium and/or selenium compound before it is pressed inside the matrix. This has the advantage of keeping the selenium and/or selenium compound inside the finished pellets at a distance from the external surfaces. The binding agent may be chosen from any tacky composition, preferably organic so as to allow its escape under the form of water vapour and CO₂ from the batch mixture by the action of heat. Water itself in small quantities can as well be used as a binding agent. Generally, the binding agent may be present in quantities of as low as 0.05% of the weight of the selenium and/or selenium compound and does not exceed a few percent of that weight. It is most often kept below 10% of that weight.

The invention also relates to the use of the pellets according to the invention for incorporating a molten glass batch in order to reduce the selenium burn off in the exhaust gas. It relates as well to a method of reducing selenium burn off in a selenium comprising glass-making process by incorporating at least part of the selenium raw material inside pellets compliant with the invention.

A last aspect of the invention is to provide a glass comprising selenium, which has been manufactured by the method detailed above. In a preferred embodiment, the glass is a soda lime glass. In another preferred embodiment, the soda lime glass comprising Se is a float glass obtained by floating a ribbon of molten glass on the upper surface of a bath of molten metal tin. The glass may also be, alternatively, any other type of glass including, non-limitatively: cast glass, drawn glass and blown cylinder glass.

Preferably, the soda lime float glass is a flat glass.

The invention will now be illustrated below by examples aiming at better describing the invention, without by no means trying to limit its scope.

EXAMPLE 1R Not in Accordance of the Invention

A first melting reference example (Example 1R) has involved a batch of raw materials comprising no pellet at all. The following solid components have been mixed in solid pulverulent form in the amount indicated:

TABLE 1 Amount, Component g Sand 139.117 Feldspar 4.744 Slag 2.116 Limestone 7.629 Dolomite 39.812 Soda Ash 42.889 Salt cake 1.763 Niter 1.539 Rouge 0.480 CoSO₄•7H₂O 0.030 Na₂SeO₃ 0.059

To those components was added 4% of the total weight of the batch of water in liquid form in order to ease the mix of the different solid components. After dry mixing of the batch solids, they have been poured into a ZAC (zirconia-alumina) crucible, the latter being inserted for 2 hours into a laboratory furnace filled with a reductive gaseous atmosphere consisting of a mixture of 95% nitrogen and 5% hydrogen, which has been preheated to 1400° C. The molten batch, which was become a molten glass, has then been casted on a cold plate and the resulting solid glass broken in small pieces. The pieces have then been inserted again in a crucible and placed in the furnace at 1400° C. The temperature was then increased up to 1450° C. at a rate of about 5° C./min. The molten glass has thereafter been casted, broken and inserted again in the furnace at 1450° C. The total duration of the stay at that temperature was about 2 hours. The temperature has thereafter been increased up to 1500° C. with the same rate of 5° C./min. The molten glass was thereafter maintained at that temperature for refining during about one hour, followed by a return to 1450° C. for thermal conditioning during about 1.5 hours.

At the end of the thermal conditioning period, the molten glass has been casted in small 4 cm×4 cm steel moulds, which were placed in an annealing furnace at 600° C. for 2.5 hours. The electric heating of the annealing furnace has thereafter been switched off and the temperature allowed to go down to ambient overnight.

The day after, glass samples were withdrawn from the moulds and their faces mechanically polished. Their chemical composition were thereafter analysed by the X-ray fluorescence technique and their remaining selenium yield computed by comparison with the amount initially added in the batch mixture.

The Se yield computation gave a result of 14%.

EXAMPLE 2R Not in Accordance of the Invention

In a second reference example (2R: mixed pellet), the composition “A” described in patent EP 0 618 177 A1 has been used for pressing one pellet that has been inserted in the middle of the raw materials solids in the same crucible as in example 1R.

The batch of raw materials and the pellet were of the following composition:

TABLE 2 Amount, Component g Batch: Sand 139.100 Feldspar 4.740 Slag 2.116 Limestone 7.629 Dolomite 39.812 Soda Ash 42.889 Salt cake 1.763 Niter 0.000 Rouge 0.469 CoSO₄•7H₂O 0.030 Na₂SeO₃ 0.000 Mixed pellet: Niter 0.840 Sand 1.840 KNO₃ 0.840 Ca(NO₃)₂•4H₂O 1.500 Na₂SeO₃ 0.059

The same procedure of melting, refining and annealing the glass has been performed as in example 1R. The measured Se yield has been, for this example, 12%.

EXAMPLES 3-5 According to the Invention

The same procedure as the one used in example 2R has been performed with modified pellets of three different natures. About half of the batch materials were carefully pressed so has to present a hollow cavity in the middle of the compressed mixture. The latter cavity has been subsequently filled with 0.059 g Na₂SeO₃ (encapsulated Se pellets) and covered with the remaining half of the batch materials. The whole pellet comprising the selenium compound in its centre has thereafter been pressed.

TABLE 3 Example 5 Example 3 Example 4 Calcined dolomite + Clinker Calcined dolomite sand Batch: Batch: Batch: Sand 139.500 Sand 139.500 Sand 136.600 Feldspar 4.740 Feldspar 4.740 Feldspar 4.740 Slag 2.116 Slag 2.116 Slag 2.116 Limestone 7.629 Limestone 7.629 Limestone 7.629 Dolomite 39.812 Dolomite 34.810 Dolomite 34.812 Soda Ash 42.889 Soda Ash 42.889 Soda Ash 42.889 Salt cake 1.763 Salt cake 1.763 Salt cake 1.763 Niter 1.539 Niter 1.539 Niter 1.539 Rouge 0.471 Rouge 0.471 Rouge 0.471 CoSO₄•7H₂O 0.030 CoSO₄•7H₂O 0.030 CoSO₄•7H₂O 0.030 Encapsulated pellet: Encapsulated pellet: Encapsulated pellet: Clinker 0.500 Calcined 0.500 Sand 0.250 Dolomite Calcined 0.250 Dolomite Na₂SeO₃ 0.059 Na₂SeO₃ 0.059 Na₂SeO₃ 0.059 Se yield 24% Se yield 28% Se yield 23%

Yields in Se have been computed and indicated at the bottom of the table. It can be seen that by encapsulating selenium compounds inside pellets can increase the yield up to 100% compared to the references. 

1. Pellets of at least 0.5 cm in their largest dimension encapsulating selenium and/or at least one Se compound, characterised in that the pellets are made of: a) one hollow cavity, located inside the body of each pellet and filled with a heart of selenium and/or at least one Se compound; b) a matrix surrounding the filled hollow cavity, which hides and isolates the heart from the pellet environment, the matrix being a compressed tablet that does not melt below 1300° C. under normal pressure, free from any compound which could give rise to a gaseous compound when heated from atmospheric temperature up to 2000° C. and which is able to slowly form an eutectic product with at least one of the constituents of a batch of molten raw materials commonly used in a glass manufacturing process, when dipped into said molten glass raw materials batch, the eutectic product having a melting temperature of from 400 to 1800° C. under normal pressure.
 2. Pellets according to claim 1, characterised in that the heart of selenium and/or at least one Se compound is selected from metallic selenium and inorganic and organic salts as well as other organic compounds comprising selenium in their molecule.
 3. Pellets according to claim 2, characterised in that the selenium compound is selected from the selenites and the selenates.
 4. Pellets according to claim 1, characterised in that the matrix comprises at least one of the following compounds: SiO₂, Na₂O, K₂O, Li₂O, B₂O₃, CaO, MgO, BaO, ZnO, Al₂O₃, Fe₂O₃, Se, TiO₂, CoO, MnO, Cr₂O₃, CeO₂, ZrO₂, NiO, SnO₂, Cu₂O, V₂O₅ and SO₃.
 5. Pellets according to claim 4, characterised in that the matrix comprises at least sand, cement, dolomite or any combination of two or more of those compounds.
 6. Pellets according to claim 1, characterised in that the matrix is able to form an eutectic with one or more of molten CaO, MgO and SiO₂ present in a raw material glass batch.
 7. Process for the manufacture of pellets according to claim 1, characterised in that: part of the total quantity of the matrix material is dry mixed the mixture is poured into a mould bearing an open cavity a rod having a cross section slightly smaller than the one of the cavity aperture is then inserted into the cavity and allowed to lay on the matrix material mixture the rod is then pressed with the aid of a press in order to solidify the matrix material into a lower half pellet the rod is removed and the selenium and/or selenium compound is deposited at the top centre area of the half pellet a second and last part of the quantity of the mixed matrix material is poured over the selenium and/or selenium compound the rod is put again in place inside the cavity of the mould and pressed the finished pellet is removed of the mold.
 8. Process for the manufacture of pellets according to claim 7, characterised in that two different rods are used, a first one with a plain section end for pressing the first lower half pellet and a second one with a small hole located in the centre of its section end in view to keep the selenium and/or selenium compound at the heart of the pellet during the pressing operation.
 9. Process for the manufacture of pellets according to claim 7, characterised in that a small proportion of a binding agent is added to the selenium and/or selenium compound.
 10. Process for the manufacture of pellets according to claim 9, characterised in that the binding agent is selected from at least one of a tacky organic composition and water.
 11. Use of the pellets according to claim 1, for incorporating a molten glass batch in order to reduce the selenium burn off in the exhaust gas.
 12. A method of reducing selenium burn off in a selenium comprising glass-making process by incorporating at least part of the selenium raw material inside pellets according to claim
 1. 13. A soda lime glass comprising selenium, which has been manufactured by the method of claim
 12. 14. Soda lime glass comprising selenium according to claim 13, characterised in that it is a float glass obtained by floating a ribbon of molten glass on the upper surface of a bath of molten metal tin.
 15. A flat glass comprising selenium, characterised in that it is the float glass of claim
 14. 